Unique, difficult-to-revert, short in-frame deletions have been detected within p6gag from clinical isolates taken from three HIV-1-infected long term nonprogressors. It is currently not known whether those deletions impart an attenuated phenotype to HIV-1. In the proposed studies, we will use SIV and the rhesus monkey model to define the relative importance and functional contribution of defined segments of p6gag to replication in cell culture and to the ability to cause AIDS in monkeys. Serial, overlapping, two amino acid deletions will be created in p6gag of SIVmac239. Replication efficiency of these mutants will then be analyzed in multiple cell lines. Rhesus monkeys will be infected with in vitro replication competent mutant strains to assess replication efficiency in vivo. Plasma viral load, PBMC viral burden and CD4 counts will be serially measured post infection. The p6gag region will also be sequenced serially over time to monitor for reversion. Should any p6gag deletion mutants demonstrate wild type replication in vitro with an attenuated phenotype in vivo, the contribution of the p6gag deletion to attenuation will be further investigated. If a contiguous stretch of four or more amino acids are dispensable for replication in cell culture, we will make and evaluate larger deletion mutations similarly. Increased knowledge of the function of p6gag may be useful for design of anti-retroviral therapy as well as the engineering of attenuated vaccine strains of SIV.